>>16481797
~28,000 years {1}. But our evolution has accelerated 100x since the agricultural revolution some odd 10,000 years ago {2} so expect that to only be 280 years today. 280/30 ~ 9 generations assuming no intermixing.

{1}.https://pmc.ncbi.nlm.nih.gov/articles/PMC3820762/

{2}.https://www.medicalnewstoday.com/articles/321942

>>16481797
It depends because there are different melanin systems with different adaptability

It isn't time based, intelligent people will be closer to the poles than equator because Vitamin D production correlates with intelligence, people who can absorb enough sunlight further north AND survive the harsh environment will naturally have less of pigment that blocks sunlight.
Of course this doesn't work if you send already retarded shitskins north and take care of them the way we care of common pets like dogs, it has to be organic.